1. Field of the Invention
The present invention relates to a multilayer ceramic substrate and a method for producing the multilayer ceramic substrate. In particular, the present invention relates to a multilayer ceramic substrate produced by a non-shrinkage process, the multilayer ceramic substrate having a cavity.
2. Description of the Related Art
When a multilayer ceramic substrate is produced, a green laminate to be formed into a multilayer ceramic substrate is fired. When such a firing step is performed, the laminate inevitably shrinks. In particular, the occurrence of uneven shrinkage prevents a higher density of wiring in a multilayer ceramic substrate. Thus, it is important that the laminate does not shrink unevenly in the firing step.
To prevent the occurrence of such uneven shrinkage, a method for producing a multilayer ceramic substrate called a non-shrinkage process has been used. When a multilayer ceramic substrate is produced by the non-shrinkage process, constraining layers are arranged so as to sandwich a green laminate to be formed into a multilayer ceramic substrate. The constraining layers include an inorganic material powder that is not sintered at a firing temperature. Thus, the constraining layers prevent the shrinkage of the laminate in the firing step, and thus, prevent the occurrence of uneven shrinkage. The constraining layers are removed after the firing step.
However, when a multilayer ceramic substrate having a cavity is produced, even when the non-shrinkage process described above is used, the cavity, in particular, the periphery of the bottom of the cavity may be undesirably deformed so as to cause cracks and breaks in conductive lines in the multilayer ceramic substrate. The reason for this is as follows. Since the constraining layers are arranged on surfaces of a green laminate, the shrinkage-inhibiting function of the constraining layers is reduced as the distance from the opening of the cavity increases. Thus, the shrinkage rate is maximized at the farthest location from the opening, i.e., at the bottom of the cavity, which causes stress concentration. With respect to the depth of the cavity, the shrinkage rate at the bottom of the cavity is increased as the depth of the cavity increases.
To solve this problem, Japanese Unexamined Patent Application Publication No. 2003-273513 discloses that a constraining layer, i.e., a constraining interlayer, is arranged at an interface between green base-material layers defining a wall of a cavity of a green laminate. The constraining interlayer is not removed after the completion of a firing step. However, the constraining interlayer is solidified by the penetration of a portion of materials included in the base-material layers and remains in the completed multilayer ceramic substrate. The arrangement of the constraining interlayer in the wall of the cavity suppresses deformation, the formation of cracks, and breaks in conductive lines at the bottom of the cavity at which the shrinkage-inhibiting effect is reduced.
For example, when a multilayer ceramic substrate having a deep cavity is produced, even when the method described in Japanese Unexamined Patent Application Publication No. 2003-273513 is used, the shrinkage-inhibiting effect is insufficient at the bottom of the cavity. This may disadvantageously cause deformation, the formation of cracks, and breaks in the conductive lines at the periphery of the bottom of the cavity on which a shrinkage stress concentrates. To solve the foregoing problems, the shrinkage-inhibiting effect should be increased.
To increase the shrinkage-inhibiting effect of the constraining interlayer, an increase in the thickness of the constraining interlayer or an increase in the number of the constraining interlayers would appear to be effective. However, in the former, an inorganic material powder included in the constraining interlayer is densified and solidified by the penetration of glass and other materials contained in the base-material layers. Thus, the constraining interlayer should have a thickness to the extent that the inorganic material powder included in the constraining interlayer is solidified by the penetration of glass and other materials included in the base-material layers. This limits the amount that the thickness of the constraining interlayer can be increased. In fact, Japanese Unexamined Patent Application Publication No. 2003-273513 discloses a thickness of about 1 μm to about 2 μm. In the latter, also as described in Japanese Unexamined Patent Application Publication No. 2003-273513, when the constraining interlayers are arranged at all interfaces between the base-material layers, a reduction in the sinterability of the base-material layers is significant.